Gmail double check blue like Whatsapp to know if they have read the emails. DoubleTick works through a Chrome extension, but you need to sign up for a free account to use it.
So just download the Chrome extension from the link at the bottom of the article and create a free account to have the double blue checkmarks for read confirmation on Gmail.
Gmail double blue check how it works
When composing an email, the tracker is added automatically. It also adds a subtitle '- Sent via Doubletick' at the end of your email which you can delete if you want.
When emails are sent and delivered to the recipient, two black dashes appear next to the email.
When the recipient opens i.e. reads / sees the email, the signs turn blue.
DoubleTick works in your browser via the Chrome extension. The recipient will not need to install the extension.
DoubleTick has access to all your email messages. The service accesses it in good faith and promises not to sell the data or share it with anyone else. It is understandable that users may be reluctant to use this service but I assure you that there is no cause for concern.
Install DoubleTick from the Chrome Web Store